Control system for vehicle automatic transmission

ABSTRACT

A control system of a lockup clutch of a torque converter of a vehicle automatic transmission. A basic manipulated variable is determined in response to the vehicle operating condition in accordance with a predetermined characteristic, and the lockup clutch engaging force is controlled in response to the variable. In the system, fuzzy reasoning is carried out using the detected vehicle operating parameters to correct the basic manipulated variable, and the engaging force is controlled in response to the corrected manipulated variable, when the control condition is met. The corrected manipulated variable is gradually decreased with respect to time when the vehicle driving state has shifted from a region in which the engaging force is controlled in response to the corrected manipulated variable to a region in which the lockup clutch is disengaged. In addition, the corrected manipulated variable is gradually increased when the vehicle driving state has shifted from a region in which the lockup clutch is disengaged to a region in which the engaging force is controlled in response to the corrected manipulated variable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a control system for a vehicle automatictransmission, more particularly to a control system for the lockupclutch of a torque converter of a vehicle automatic transmission.

2. Description of the Prior Art

Automatic transmissions are usually designed to automatically select theoptimum gear by detecting vehicle or engine speed and the degree ofthrottle opening or some other such parameter indicating engine load andthen retrieving a gear by the detected values of these parameters fromgear shift characteristics (a shift map) determined in advance based onthe same parameters.

In the automatic transmission of this type, a hydraulic torque converterequipped with a lockup clutch is installed between the internalcombustion engine (power source) and the transmission unit. The torqueincrease characteristic of the torque converter is utilized when thevehicle is accelerating, as during drive-away or overtaking, while thelockup clutch completely or partially engages the input and output sidesof the hydraulic torque converter during cruising. The ON (operative)and OFF (inoperative) regions of the lockup clutch are defined inadvance in terms of the gear (gear ratio) and other driving conditionsand the lockup clutch is controlled based thereon to preventtransmission efficiency loss.

Because a type of surging known as body vibration may occur in theacceleration region when the operator steps down on the acceleratorpedal, drivability considerations make it extremely difficult tooptimize the ON/OFF characteristics of the lockup clutch. The prior artpractice has therefore been to keep the lockup clutch normally OFF whenin the acceleration region is entered owing to depression of theaccelerator pedal or, more precisely, to define this region as a weaklockup region in which only a weak engagement determined from thecharacteristics of the hydraulic circuit is imparted. A description ofthis prior art technology can be found, for example, in JapaneseLaid-Open Patent Application No. Sho 63(1988)-180,757.

The need to expand the ON (operative) region of the lockup clutch hasheightened in recent years, however, owing to increased demand forlesser fuel consumption. The prior art cannot sufficiently respond tothis requirement. On the other hand, the ON (operative) region cannot beindiscriminately broadened because, as just pointed out, this would leadto surging and degrade drivability.

An object of this invention is therefore to overcome the aforesaidproblems by providing a control system for a vehicle automatictransmission which expands the ON (operative) region of the lockupclutch and achieves improved fuel efficiency while avoiding theoccurrence of surging.

SUMMARY OF THE INVENTION

Another object of this invention is to provide a control system for avehicle automatic transmission which expands the ON (operative) regionof the lockup clutch and achieves improved fuel efficiency and which,contrary to what might be expected, simultaneously provides animprovement in drivability with regards to direct control feel and theresponse of vehicle speed to accelerator pedal depression in theacceleration region.

This invention achieves this object by providing a system forcontrolling an automatic transmission of a vehicle, comprising, couplingmeans (clutch means) having an input connected to an internal combustionengine mounted on the vehicle and an output connected to a gear systemin the transmission, said coupling means passing engine power to thegear system, gear ratio establishing means for establishing a gear ratioof the gear system in response to a gear shift command to transmit theengine power to a vehicle wheel through the established gear ratio,engaging force control means for controlling the engaging force of thecoupling means including at least an engaging state and a disengagingstate, vehicle operating condition detecting means for detectingoperating conditions of the vehicle, and basic manipulated variabledetermining means for determining a basic manipulated variable to beapplied to said engaging means in response to a parameter of thedetected vehicle operating conditions in accordance with a predeterminedcharacteristic. In the system, said engaging force control meansincludes, fuzzy reasoning means for carrying out fuzzy reasoning using aparameter of the detected operating conditions of the vehicle todetermine a correction value of the basic manipulated variable, andmanipulated variable correcting means for correcting the basicmanipulated variable based on the correction value. And in the system,said engaging force control means controls the engaging force of thecoupling means in response to the corrected manipulated variable.

BRIEF EXPLANATION OF THE DRAWINGS

This and other objects and advantages of the invention will be moreapparent from the following description and drawings, in which:

FIG. 1 is an overall schematic view showing the configuration of acontrol system for a vehicle automatic transmission;

FIG. 2 is a block diagram showing a hydraulic control of a lockup clutchillustrated in FIG. 1;

FIG. 3 is a flowchart showing the operation of the system according tothe invention;

FIG. 4 is a graph showing operational regions of the lockup clutchillustrated in FIG. 1;

FIG. 5 is a subroutine of the FIG. 3 flowchart showing the determinationof a manipulated variable to be applied to the lockup clutch throughfuzzy reasoning;

FIGS. 6 to 10 are explanatory views showing fuzzy production rules usedin the fuzzy reasoning of FIG. 5;

FIG. 11 is a graph showing the characteristic of a basic duty valueBDUTY (basic manipulated variable) used in the procedure of FIG. 5;

FIG. 12 is an explanatory view showing the fuzzy reasoning of FIG. 5;

FIG. 13 is a flowchart showing another operation of the system accordingto a second embodiment of the invention;

FIG. 14 is a timing chart explaining the operation of FIG. 13;

FIG. 15 is a flowchart showing still another operation of the systemaccording to a third embodiment of the invention;

FIG. 16 is a timing chart explaining the operation of FIG. 15; and

FIG. 17 is another timing chart similarly explaining the operation ofFIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will now be explained with reference tothe attached drawings.

FIG. 1 is an overall view of a control system for a vehicle automatictransmission according to the invention.

As shown in FIG. 1, a crankshaft 10 of an internal combustion engine Eis connected with a main shaft MS, through a hydraulic torque converter(hydraulic coupling means) 12 having a lockup clutch L (coupling means),of a vehicle automatic transmission T. The automatic transmission T hasa gear system made up of the main shaft MS, a countershaft CS and asecondary shaft SS which are arranged in parallel and support gearsthereon.

More specifically, the main shaft MS supports a main first-speed gear14, a main third-speed gear 16, a main fourth-speed gear 18 and a mainreverse gear 20. The countershaft CS supports a counter first-speed gear22 engaged with the main first-speed gear 14, a counter third-speed gear24 engaged with the main third-speed gear 16, a counter fourth-speedgear 26 engaged with the main fourth-speed gear 18 and a counter reversegear 30 engaged with the main reverse gear 20 through a reverse idlegear 28. The secondary shaft SS supports a first secondary second-speedgear 32 and a second secondary second-speed gear 34.

The first gear is established when the main first-speed gear 14rotatably supported on the main shaft MS is connected with the mainshaft MS by a first-speed hydraulic clutch C1. Since the first-speedhydraulic clutch C1 is also maintained in the engaged state duringestablishment of the second-fourth gears, the counter first-speed gear22 is supported by a one-way clutch COW. A hydraulic clutch CLH isprovided for holding the first gear so as to ensure that the drivenwheels W drive the engine E, i.e., to ensure the engine braking effectwhen 1st or 2nd range (explained later) is selected.

The second gear is established, through the main third-speed gear 16,the counter third-speed gear 24 and the first secondary second-speedgear 32, when the second secondary second-speed gear 34 rotatablysupported on the secondary shaft SS is connected with the secondaryshaft SS by a second-speed hydraulic clutch C2. The third gear isestablished when the counter third-speed gear 24 rotatably supported onthe countershaft CS is connected with the countershaft CS by athird-speed hydraulic clutch C3.

The fourth gear is established when the counter fourth-speed gear 26rotatably supported on the countershaft CS is connected with thecountershaft CS by a selector gear SG and with this state maintained themain fourth-speed gear 18 rotatably supported on the main shaft MS isconnected with the main shaft MS by a fourth-speed/reverse hydraulicclutch C4R. The reverse gear is established when the counter reversegear 30 rotatably supported on the countershaft CS is connected with thecountershaft CS by the selector gear SG and with this state maintainedthe main reverse gear 20 rotatably supported on the main shaft MS isconnected with the main shaft MS by the fourth-speed/reverse hydraulicclutch C4R.

The rotation of the countershaft CS is transmitted through a final drivegear 36 and a final driven gear 38, engaged therewith, to a differentialD, from where it is transmitted to the driven wheels W through left andright drive shafts 40 (only one shown).

A throttle position sensor S1 is provided in an air intake pipe (notshown) of the engine E at a point in the vicinity of a throttle valve(not shown) for detecting the degree of opening or position θTH of thethrottle valve. A vehicle speed sensor S2 for detecting the vehicletraveling speed V from the rotational speed of the final driven gear 38is provided in the vicinity of the final driven gear 38. A transmissioninput shaft speed sensor S3 is provided in the vicinity of the mainshaft MS for detecting the rotational speed NM of the transmission inputshaft from the rotation of the main shaft MS, and a transmission outputshaft speed sensor S4 is provided in the vicinity of the countershaft CSfor detecting the rotational speed NC of the transmission output shaftfrom the rotation of the countershaft CS.

A shift lever position sensor S5 is provided in the vicinity of a shiftlever (not shown) installed on the vehicle floor near the driver's seat.The shift lever position sensor S5 detects which of the seven ranges orpositions P, R, N, D4, D3, 2, 1 has been selected by the driver. A crankangle sensor S6 is provided in the vicinity of the crankshaft 10 of theengine E for detecting the engine speed Ne from the rotation of thecrankshaft 10, and a coolant temperature sensor S7 for detecting theengine coolant temperature Tw is provided at an appropriate location ona cylinder block (not shown) of the engine E. Outputs of the sensors S1,etc., are sent to an ECU (electronic control unit).

The ECU is constituted as a microcomputer comprising a CPU (centralprocessing unit) 50, a ROM (read-only memory) 52, a RAM (random accessmemory) 54, an input circuit 56 and an output circuit 58. The outputs ofthe sensors S1, etc., are input to the microcomputer through the inputcircuit 56. The CPU 50 of the microcomputer conducts gear shift controlincluding the lockup clutch control and issues a command to a hydrauliccontrol circuit O via the output circuit 58.

The hydraulic control circuit O has shift solenoids SL1, SL2, an ON/OFFcontrol solenoid SL3 and a capacity (engaging force) control solenoidSL4 for the lockup clutch L, and a linear solenoid SL5 for regulatingclutch oil pressure. More specifically, the CPU 50 determines the shiftposition (gear ratio) based on the outputs of the sensors andenergizes/deenergizes the shift solenoids SL1, SL2 of the hydrauliccontrol circuit O via the output circuit 58 so as to switch shift valves(not shown) and thereby engage/disengage the hydraulic clutches ofprescribed gears. As will be explained later, it also controls thelockup clutch in the ON (fully engaged or operative) state, or in theOFF (fully disengaged or inoperative) state, or in a partially engagedor slip-controlled state between these two states such that the torqueconverter 12 is slip-engaged.

The torque converter 12 comprises a pump 12a connected to the crankshaft10, a turbine 12b connected to the main shaft MS, a stator 12c and thelockup clutch L. The lockup clutch L is of the known configurationcomprising a lockup piston, a damper spring, etc. Depending on theamount of oil pressure supplied to its left and right chambers, thelockup clutch assumes the ON state (shown by solid lines in FIG. 2discussed below), the OFF state (shown by phantom lines in FIG. 2) orthe slip-controlled state.

When the lockup clutch is ON, the power of the engine E is transmittedto the main shaft MS through a drive plate, a torque converter cover andthe lockup clutch L. When the lockup clutch is OFF, the engine power istransmitted to the main shaft MS through the drive plate, the torqueconverter cover, the pump 12a and the turbine 12b.

FIG. 2 is a block diagram functionally illustrating the hydraulicoperation of the lockup clutch L. The lockup clutch is turned ON and OFFby supplying or not supplying line pressure from a manual valve to alockup shift valve which receives modulator pressure from a modulatorvalve through the solenoid SL3. An L/C (lockup) control valve whichreceives the modulator pressure (through the solenoid SL4) controls theengaging force of the lockup clutch L by regulating the oil pressuresupplied to the right chamber of the clutch. The torque converter 12 isthus controlled to the aforesaid completely locked-up (fully-engaged orON) state by an L/C (lockup) timing valve which receives throttlepressure through the linear solenoid (throttle valve) SL5 and themodulator pressure (through the solenoid SL4).

FIG. 3 is a flowchart of the operations of a control system for avehicle transmission according to this invention. Prior to going intothe details of this flowchart, however, a background explanation willfirst be given with reference to FIG. 4 regarding the lockup clutchoperating characteristics (hereinafter referred to as the LC map) in theinvention system. The characteristics shown in FIG. 4 are definedrelative to throttle opening θTH and vehicle speed V.

The reference symbols 1, 2 and 3 in this figure designate the switchoverlines of the aforesaid solenoid SL3 and solenoid SL4. To the right ofline 3 in this figure, the solenoid SL3, which conducts two position(engagement/disengagement) control of the lockup clutch, and thesolenoid SL4, which controls the engaging force therebetween based onthe desired slip ratio of the torque converter, are both ON. In thecontrol of the engaging force, the duty value (PWM duty value or ratio)applied to the solenoid SL4 as the manipulated variable is calculatedbased on the desired slip ratio (amount) of the torque converter.

More specifically, on the right side of the figure the region (a) is acompletely locked-up region in which a duty value of 100% is applied tothe solenoid SL4 for directly connecting the input and output sides ofthe torque converter. The region (b) is a strong lockup region in whichthe duty value of the solenoid SL4 is increased from the value in theadjacent region toward 100% in prescribed increments so as to graduallyincrease the engaging force and decrease the slip ratio. The region (c)is a region of relatively stable driving conditions involving littlefluctuation in engine speed. In this region, the duty value forobtaining the desired slip ratio is learned by using a PI controller toconduct feedback control based on the error between the desired slipratio and the actual slip ratio.

The region (d) comprises a hatched portion α and a region β lyingoutside the hatched portion. In the prior art systems, the solenoid SL4is turned OFF in the region β outside the hatched portion and the lockupclutch is turned OFF in hatched portion α. In the invention, on theother hand, the solenoid SL4 is left ON in the region (d) and theengaging force is duty-controlled (PWM controlled). As a specificexample of prior-art system operation, take the case where, as indicatedby the arrows a in FIG. 4, the operating state moves from the region (c)or the like, across the line 3 and into the region β outside the hatchedportion of the region (d) owing to a decrease in the vehicle speed V oran increase in the throttle opening θTH. In this case, the solenoid SL3remains ON, but since the solenoid SL4 is turned OFF, the lockup clutchis applied with only the minimum capacity determined from thecharacteristics of the hydraulic circuit, so that only the weak engagingforce at this lower limit is applied. Then when the operating statemoves across the line 2, the solenoid SL3 is turned OFF and the lockupclutch becomes inoperative.

Thus the prior art systems cope with the risk that even a slightvariation in throttle opening may lead to surging in the region (d) bylimiting the engaging force to not more than that dictated by the systemhardware. In contrast, this invention enables the engaging force to beincreased by determining the duty value through fuzzy reasoning in themanner described hereinafter.

The region (e) is a deceleration lockup region. Since variation in thetorque from the engine E is not a problem in this region, feedbackcontrol employing a PI controller is controlled for obtaining the dutyvalue needed to secure the desired slip ratio of the torque converter(=NM/Ne×100%; explained later), namely, a slip ratio in the range of 102to 103%, thereby ensuring a good engine braking effect. In the region(f), the lockup clutch is disengaged, in both the prior art and theinvention.

The operation of the invention system will now be explained withreference to the flowchart of FIG. 3, taking as an example the casewhere the operating state passes over the line 3 from the right asindicated by the arrows a in FIG. 4. The routine of FIG. 3 is actuatedat appropriate time intervals of, for example, 20 ms.

First, in S10, it is checked whether the ordinary lockup clutch (LC)operating conditions are met. Specifically, it is checked whether theengine coolant temperature Tw, engine speed Ne, vehicle speed V andthrottle opening θTH are within prescribed ranges and that systemfailure has not occurred.

When the result in S10 is NO, the routine is immediately terminated.When it is YES, the program goes to S12, in which it is checked whetherthe selected range is D. When it is not, the program goes to S14, inwhich control for the selected range is conducted, and when it is, itgoes to S16, in which it is checked whether the lockup clutch is inoperation (ON state). In other words, it is confirmed whether theoperating state is moving in the direction of the arrows a in FIG. 4,not in the direction of the arrows b.

This step is necessary because structural (hardware) differences betweendifferent lockup clutches, including their hydraulic circuits, make itimpossible to achieve the desired torque converter slip ratio even iffine engaging force control is started immediately after the lockupclutch has been disengaged. Conversely, good control performance can beobtained when the lockup clutch is in operation, namely, when thesolenoids SL3 and SL4 are ON.

More specifically, the general tendency is for the engaging force of thelockup clutch and the slip ratio of the torque converter to decrease asthe operating state moves from right to left in FIG. 4, and the controlperformance is better in the direction of decreasing engaging force andslip ratio. When the result in S16 is NO, therefore, the program goes toS18, in which ordinary D-range lockup clutch control is conducted.Specifically, SL4 is turned OFF and weak lockup control determined bythe hardware is conducted.

On the other hand, when the result in S16 is YES, the program moves toS20, in which it is checked whether the vehicle is currently travelingover level ground. This step is conducted because the driven wheels Wmay drive the engine E depending on the slope of an uphill or downhillgrade, and it affects the torque converter slip ratio and the lockupclutch engaging force and makes the probability of surging occurrencehigh.

Discrimination of whether the road is level can be conducted by using aninclination sensor mounted at an appropriate location on the vehicle orby adopting the technique taught by the assignee's Japanese Laid-OpenPatent Application No. Hei 6(1994)-109,122 (filed in the United Statesand patented under the number of U.S. Pat. No. 5,317,937) of using anindex indicative of the running resistance calculated from the vehicleacceleration to select from multiple maps for level-road driving,hill-climbing, etc., prepared in advance and making the discriminationbased on whether or not an LC map for level-road driving is selected.

When the result in S20 is NO, the program goes to S18, in which controlis conducted in the same manner as in the prior art, and when it is YES,the program goes to S22, in which it is checked whether the current gear(speed) is fourth gear. This step is conducted because a margin forsurging is greater for a higher gear (smaller gear ratio) in the Drange. When the result in S22 is NO, the program goes to S18, in whichcontrol is conducted in the same manner as in the prior art, and when itis YES, the program goes to S24, in which it is checked whether thecurrent vehicle speed V is at or below a predetermined speed of, forinstance, 50 km/h. When the result in S24 is YES, the program goes toS26, in which it is checked whether any auxiliary equipment is inoperation. By "auxiliary equipment" is meant an air conditioner or othersuch equipment driven by the output (power) of the engine E.

When the result in S26 is YES, the program goes to S18, in which controlis conducted in the same manner as in the prior art, and when it is NO,the program goes to S28, in which lockup clutch control is conductedusing fuzzy reasoning (approximate reasoning). This method is adoptedbecause the operation of auxiliary equipment at or below thepredetermined speed strongly affects the engine output and therotational speed on the input side of the torque converter 12, makingappropriate slip control of the lockup clutch difficult. When S24 findsthat the current vehicle speed is greater than the predetermined speed,the risk of fluctuation in the engine output and the rotational speed onthe input side of the torque converter 12 can be assumed to be low.Since the slip control is therefore not likely to be affected, theprogram skips S26 and goes directly to S28.

FIG. 5 is the flowchart of a subroutine for determining the manipulatedvariable for the lockup clutch control using fuzzy reasoning, and FIGS.6 to 10 are diagrams for illustrating fuzzy production rules used in thefuzzy reasoning.

As shown in FIGS. 6 to 10, the fuzzy reasoning uses ten rules whoseantecedents include as parameters the vehicle speed V, the throttleopening θTH and the torque converter slip ratio ETR. As shown in thefigures, membership functions are defined within the ranges of a vehiclespeed V between 0 and 255 km/h, a throttle opening θTH between fullyclosed and wide open, and a torque converter slip ratio ETR between 18and 120%.

The slip ratio ETR of a torque converter is ordinarily calculated as(rotational speed of turbine input shaft)/(rotational speed of pumpinput shaft). In this embodiment, however, it is calculated as(rotational speed of main shaft NM)/(rotational speed of engineNe)×100%. (The upper limit of 120% is set in consideration of the enginebraking effect.)

The parameter of the conclusion is a correction coefficient (correctioncoefficient LFK) for correcting the basic duty value (basic manipulatedvariable). As illustrated, the membership function is set between 0 and1.0. The basic duty value (indicated as BDUTY in FIG. 5 etc.) isestablished as a table (based on the characteristic curve shown in FIG.11) and is a value defining the upper limit of the duty value output tothe solenoid SL4 as a function of engine load (throttle opening θTH). Inother words, the correction coefficient LFK is obtained by fuzzyreasoning using the vehicle speed V or the like and the basic duty valueBDUTY is multiplied thereby to obtain the corrected duty value (dutyvalue FBDY) to be output to the solenoid SL4 as the control input.

As shown by the characteristic curve of FIG. 11, the basic duty value isdefined to decrease with increasing engine load (throttle opening θTH).Needless to say, the purpose of this is to counteract the higherprobability of surging with increasing throttle opening by reducing theduty value and thus lowering the engaging force of the lockup clutch.

Among the rules shown in FIGS. 6 to 10, rules 1 and 2, 3 and 4, 5 and 6,7 and 8, and 9 and 10 are established in connection with the slip ratioETR (of the torque converter) so that the membership functions of theslip ratio ETR complement each other. In order to prevent hunting,however, the membership function of the correction coefficient LFK ofthe conclusion is given hysteresis.

As driving conditions, rules 1 and 2 contemplate low vehicle speed andlarge slip ratio, rules 3 and 4 contemplate rather low vehicle speed andrather large slip ratio, rules 5 and 6 contemplate medium vehicle speedand medium slip ratio, rules 7 and 8 contemplate rather high vehiclespeed and rather small slip ratio, and rules 9 and 10 contemplate highvehicle speed and small slip ratio. Since the basic duty value is setwith respect to the throttle opening θTH, the membership function of thethrottle opening is set at 1.0 over the whole throttle opening range,meaning that the throttle opening is not actually used in the fuzzyreasoning. It should be understood, however, that it can be set asdesired if the necessity arises.

This fuzzy reasoning will now be explained with reference to theflowchart of FIG. 5. First, in S100 and S102, computation tables LHIGHand LAREA (explained later) are initialized to zero, whereafter thevalue of a counter LNRULE (which counts the number of rules) isinitialized to zero in S104.

The program then advances to S106, in which the value of the rulecounter LNRULE is incremented, to S108, in which the counter value isset to n (initial value 1), to S110, in which the value of theantecedent of rule n (in this case rule 1) is calculated, to S112, inwhich the value of the conclusion is calculated, and to S114, in whichthe value of the rule counter is compared with 10, whereafter S106 toS112 are looped until the counter value reaches 10 and all rules havebeen similarly processed.

The reasoning used is illustrated in FIG. 12. The minimum value amongthe three antecedent membership functions, the value of slip ratio ETRin the illustrated case, is used to determine the conclusion membershipfunction grade (the height Yn' on the y-axis; corresponding to theaforesaid LHIGH) and the area LAREA is then calculated as shown in thefigure. The y-axis height LHIGH of the conclusion and the area LAREA arecalculated for each loop of the procedure up to S114 and the results aretotaled. Then, in S116 and the following steps, the inferred value (thecorrection coefficient LFK) is calculated by dividing the total value ofLAREA by the total value of LHIGH to obtain the center of gravity.

More specifically, the program advances to S116, in which it is checkedwhether the y-axis height LHIGH is zero and, when it is, to S118, inwhich correction coefficient LFK is set to zero to avoid division byzero. When the result in S116 is NO, the program goes to S120, in whichthe center of gravity is calculated as explained and the so-calculatedvalue in the x-axis (universe of discourse) is defined as the correctioncoefficient. The program then goes to S122, in which it is checkedwhether the calculated correction coefficient LFK is in overflow and,when it is, to S124, in which the calculated correction coefficient LFKis rewritten to its upper limit of 1.0.

On the other hand, when the result in S122 is NO, the program goes toS126, in which the detected throttle opening θTH is used as address datato retrieve the basic duty value BDUTY from a table corresponding to thecharacteristic curve of FIG. 11 and then to S128, in which the retrievedbasic duty value BDUTY is multiplied by the correction coefficient LFKobtained by fuzzy reasoning to thereby obtain the output duty value FBDY(corrected manipulated variable). This output duty value FBDY is thenoutput to the solenoid SL4 by another routine not shown in the figures.

The foregoing is the control conducted in the region (d) of FIG. 4 when,owing to depression of the accelerator pedal or the like, the operatingstate moves across the line 2 from one in which the solenoids SL3 andSL4 are both ON and the lockup clutch is engaged. The prior art responseto this situation is to turn the solenoid SL4 OFF at the point ofcrossing the line 2 and conducted only weak lockup control in the regionβ outside the hatched portion of the region (d).

As explained in the foregoing, however, in this invention the solenoidSL4 is kept on until the line 1 is crossed. As a result, an improvementin fuel economy is achieved owing to the increased engaging force in theregion (d) (including both the hatched portion α and the region βoutside the hatched portion, as defined earlier). In addition, since thedegree of increase in the engaging force, i.e., the degree of increasein the duty value, is decided through fuzzy reasoning, the improvementin fuel economy can be achieved without giving rise to surging.Moreover, the drivability can be simultaneously improved in regard todirect control feel and the response of vehicle speed to acceleratorpedal depression in the acceleration region.

In addition, the manipulated variable is obtained by correcting thebasic manipulated variable by multiplying the correction coefficientobtained through fuzzy reasoning. It is thus easy to introduce theresult of the fuzzy reasoning in the manipulated variable and to adjustthe value with the manipulated variable obtained without conductingfuzzy reasoning, making the system configuration simple.

The foregoing describes the control when the operating state moves inthe direction of the arrows a in FIG. 4.

Next, the control when the operation state moves in the direction of thearrows c in FIG. 4 will be explained, as a second embodiment of theinvention.

This is the control conducted at the time of moving from the region (d)into the region (f), namely, at the time of shifting from engaging forcecontrol using fuzzy reasoning (i.e., control based on the correctedmanipulated variable) to control with the lockup clutch inoperative.

One case in which this state arises is when the vehicle speed Vdecreases or the engine load increases (accelerator pedal depression).Another is when the technique taught by the assignee's earlier mentionedJapanese Laid-Open Patent Application No. Hei 6(1994)-109,122 (U.S. Pat.No. 5,317,937) is adopted and an LC map other than one for level-roaddriving is selected. More specifically, control for increasing theengaging force by fuzzy reasoning is conducted only when the LC map forlevel-road driving is in use. If during driving in the region (d) usinga map for level-road driving, the vehicle should begin hill-climbing orhill-descent, for example, the LC map is switched to the ordinary LC mapfor the D range. In the case of the ordinary LC map at this time, thehatched portion α of the region (d) is a region in which the lockupclutch is inoperative. The switching of the LC map thus causes a shiftfrom engaging force control using fuzzy reasoning to control with thelockup clutch inoperative. In this embodiment, when a control shiftoccurs in either of these two ways and, in addition, the operating statestays in the region (f) without a gear shift (with the transmission keptin fourth gear), the output duty value is gradually reduced to preventthe shock that would be caused by suddenly disengaging the lockupclutch.

This control will now be explained with reference to the flowchart ofFIG. 13. Like the routine of FIG. 3, this routine is also actuated atappropriate time intervals of, for example, 20 ms.

First, in S200, it is checked whether the operating state is in a lockupclutch disengaged region, namely, whether it is in the region (f). Whenthe result is NO, the routine is immediately terminated. When it is YES,the program goes to S202, in which it is checked whether the operatingstate was in a fuzzy control region, i.e., the region (d), in thepreceding cycle. If the result is NO, the routine is immediatelyterminated. If it is YES, the program goes to S204, in which it ischecked whether gear shift has occurred. This check is made because ifgear shift has occurred the lockup clutch has to be promptly disengagedin order to prevent an accompanying shock.

If the result in S204 is NO, the program goes to S206, in which it ischecked whether the output duty value FBDY is zero. If the result isYES, this means that engaging force decrease control need not beconducted and the routine is immediately terminated. If the result isNO, the program goes to S208, in which a duty value reduction amountDelta d is calculated. The value of the reduction amount Delta dincreases with increase in the slip ratio ETR of the torque converter orwith the rate of change in the engine load (specifically the differencevalue Delta θTH in the detected throttle opening θTH between thepreceding and current cycles). This relationship is established becauseof the need to rapidly disengage the lockup clutch to prevent shock whenthe slip ratio is large. The program then goes to S210, in which, asshown in FIG. 14, the calculated reduction amount Delta d is subtractedfrom the output duty value FBDY (corrected manipulated variable) and thevalue obtained is defined as output duty value DUTY. (This duty value isthen output to the solenoid SL4 by another routine not shown in thefigures.) The aforesaid processing is repeated until S206 finds that theoutput duty value FBDY has become zero.

This control according to the flowchart of FIG. 13 imparts an engagingforce in the region (d) that is greater than the weak lockup and thenenables a smooth, shock-free transition to the lockup clutch disengagedregion (f).

An explanation will now be made regarding a third embodiment of theinvention that relates to a separate auxiliary control, namely, to acontrol for increasing the engaging force by use of fuzzy reasoning whenthe operating state moves from the region (f) to the region (d) in FIG.4. Since, as mentioned earlier, engaging force increase control usingfuzzy reasoning is not conducted at the time of a direct shift fromregion (f) to the region (d), owing to control performanceconsiderations, this control is, more precisely speaking, that in thecase where the operating state moves from the region (f) through theregion (d) to the region (c) and then immediately back to the region(d).

This control will now be explained with reference to the flowchart ofFIG. 15. This routine is also actuated at appropriate time intervals of,for example, 20 ms.

First, in S300, it is checked whether the operating state is in a fuzzycontrol region, specifically, the region in which the correctedmanipulated variable is used in the control, more specifically theregion (d). If the result is NO, the routine is immediately terminated.If the result is YES, the program goes to S302, in which it is checkedwhether the elapsed time period T from the establishment of a lockupclutch operative state in which the control using fuzzy reasoning is notconducted, such as in the region (c), exceeds a prescribed time periodT1. The reason for this is that owing to response delay of the hydrauliccircuit, etc., of the lockup clutch the lockup clutch does notinstantaneously follow the anticipated state when an output duty valueis calculated by fuzzy reasoning and sent to the solenoid SL4 as acommand.

In other words, there is a risk that the feedback control of theengaging force based on the desired slip ratio will not be accuratelyconducted. If, for instance, the hydraulic circuit cannot actuallyfollow a command to increase the engaging force, the engaging force willremain unchanged and engine revving may occur. If feedback control isconducted under this condition, the engaging force is increased evenfurther, making it likely that the lockup clutch will suddenly engageand produce shock. In the case of moving from the inoperative(disengaged) state of the lockup clutch to the operative (engaged state)thereof (except in the region (d)), it is checked whether the timeelapsed since entry into the region (c) is equal to or greater than theprescribed time period T1. (See FIG. 16.)

When the result in S302 is YES, the program goes to S304, in which it ischecked whether the slip ratio ETR is between an upper limit value ETRH(110%, for example) and a lower limit value ETRL (80%, for example).This check is made to avoid inaccurate control and engine revving whichmay occur owing to the fact that the control performance is poor evenafter the elapse of the prescribed time period T1 if the slip ratio ETRof the torque converter is not in the prescribed range (80-110%). Whenthe result in S304 is YES, since it can be concluded that the prescribedtime period T1 has passed and the control performance is good, theprogram goes to S306, in which the output duty value FBDY calculated byfuzzy reasoning is defined as the output duty value DUTY.

On the other hand, if S302 finds that the prescribed time period T1 hasnot passed, the program goes to S308, in which a timer (down counter) T2is set and started, to S310, in which it is checked whether a secondtime period T2 has passed, i.e., whether the time value has reachedzero. Since the timer was just set in the preceding step, the resulthere is naturally NO and the program goes to S312, in which the outputduty value DUTY is set to 0%.

The reason for setting the output duty value to 0% instead of to theduty value calculated just before crossing the line 3 in FIG. 4 is thatif the duty value calculated just before crossing the line 3 should beoutput at the point of entering the fuzzy reasoning region, the dutyvalue would thereafter actually be decreased to a lower value calculatedby fuzzy reasoning. During rapid change of the throttle opening in therelatively low vehicle speed region where surging is a particularproblem, the probability of surging occurrence is high when the dutyvalue calculated just before crossing the line 3 is output.

In this embodiment, therefore, the duty value is first set to 0% andthen gradually increased to the value calculated by fuzzy reasoning.Rather than setting the duty value to 0%, it is also possible whencontrol response is a concern to set it to a low value smaller than theduty value calculated just before crossing the line 3.

When S304 in FIG. 15 finds that the slip ratio ETR is not in theprescribed range, the program goes to S310 and if the result here is NO,the same processing as just described is conducted. Then when S310 findsthat the time period T2 has passed, the program goes to S314, in whichthe duty value is gradually increased from 0% (or a low value) to theoutput duty value FBDY calculated by fuzzy reasoning.

In other words, as shown in FIG. 17, when the line 3 is crossed from theinoperative (disengaged) state of the lockup clutch before the firsttime period T1 has passed, or when a control performance degradesbecause the slip ratio is not in the prescribed range, the duty value of0% (or a low value) is maintained for the second time period T2 and isthereafter gradually increased to the output duty value FBDY calculatedby fuzzy reasoning. As a result, accurate feedback control can beachieved.

Since this embodiment is configured in the foregoing manner, thesolenoid SL4 is not turned OFF until the line 1 is crossed. As a result,an engaging force greater than that of weak lockup conducted in theprior art is applied in the region (d), thereby enabling an improvementin fuel economy over the prior art.

In addition, since the degree of increase in the engaging force, i.e.,the degree of increase in the duty value, is determined through fuzzyreasoning, the improvement in fuel economy can be achieved withoutgiving rise to surging and thus without degrading drivability. To thecontrary, the increase in engaging force improves the drivability byenhancing direct control feel and the response of vehicle speed toaccelerator pedal depression in the acceleration region.

Further, since the duty value is gradually raised/decreased and theengaging force is gradually increased/decreased at movement from thefuzzy reasoning region (d) into the lockup clutch disengaged region (f)and at movement from the lockup clutch disengaged region (f) immediatelyinto the fuzzy reasoning region (d) via the region (c) or the like, nosudden engagement/disengagement of the lockup clutch occurs. Thetransition can therefore be made smoothly without producing shock.

Moreover, at movement from the lockup clutch disengaged region (f) intothe fuzzy reasoning region (d) via the region (c) or the like, a dutyvalue calculated by fuzzy reasoning is used only after elapse of thetime period T1 and when the slip ratio is within a predetermined range,and in other cases the duty value of 0% (or the low duty value) is helduntil the second time period T2 has passed and is thereafter graduallyraised to the duty value calculated by fuzzy reasoning. As a result,problems such as engine revving do not occur. And since accuratefeedback control is conducted, shock and other problems caused by suddenincrease in engaging force are also nonexistent.

While the foregoing description assumes the slip state of the torqueconverter to be ascertained in terms of slip ratio, it can instead beascertained in terms of slip amount.

While the invention has thus been shown and described with reference tothe specific embodiments, it should be noted that the invention is in noway limited to the details of the described arrangements, and changesand modifications may be made without departing from the scope of theappended claims.

What is claimed is:
 1. A system for controlling an automatictransmission of a vehicle, comprising:hydraulic coupling means having aninput connected to an internal combustion engine mounted on the vehicleand an output connected to a gear system in the transmission, saidhydraulic coupling means passing engine power to the gear system; gearratio establishing means for establishing a gear ratio of the gearsystem in response to a gear shift command to transmit the engine powerto a vehicle wheel through the established gear ratio; a clutch forengaging/disengaging the input and the output of the hydraulic couplingmeans; engaging force control means for controlling the engaging forceof the clutch; vehicle operating condition detecting means for detectingoperating conditions of the vehicle; and basic manipulated variabledetermining means for determining a basic manipulated variable to beapplied to said clutch in response to a parameter of the detectedvehicular operating conditions in accordance with a predeterminedcharacteristic set with respect to at least a parameter indicative ofengine load; wherein said engine force control means includes: fuzzyreasoning means for carrying out fuzzy reasoning based on the detectedoperating conditions of the vehicle to determine a correction value ofthe basic manipulated variable; manipulated variable correcting meansfor correcting the basic manipulated variable based on the correctionvalue; and discriminating means for discriminating whether a controlcondition for engaging force control based on the corrected manipulatedvariable is met; and wherein said engaging force control means controlsthe engaging force of the clutch in response to the correctedmanipulated variable, when the control condition is discriminated to bemet.
 2. A system according to claim 1, wherein the control condition iswhen the vehicle travels over level ground.
 3. A system according toclaim 1, wherein the control condition is when an auxiliary equipmentthat uses the engine power is in operation.
 4. A system according toclaim 1, wherein the control condition is when the established gearratio is small.
 5. A system according to claim 1, wherein said engagingforce control means controls the engaging force of the clutch inresponse to the corrected manipulated variable when the controlcondition is discriminated to be met, if the vehicle is in apredetermined driving state.
 6. A system according to claim 1, whereinsaid basic manipulated variable determining means determines the basicmanipulated variable in response to the throttle opening.
 7. A systemaccording to claim 1, wherein said fuzzy reasoning means carries outfuzzy reasoning using at least one parameter among the throttle opening,vehicle speed and slip ratio of said hydraulic coupling means.